1. Field
The instant invention relates to ceramics strengthened through the inclusion of one or more compressive stress zones within a ceramic body.
2. State of the Art
The strengthening of materials which fail primarily in tension by the introduction of compressive stress zones within the body is well known in the field of glasses. Compressive stresses have been introduced into the surface areas of glass for a long time by the technique of thermal tempering wherein the surface of the glass is cooled rapidly while the glass is quite hot. At the temperatures involved in thermal tempering, the glass is sufficiently fluid that the surfaces may be cooled rapidly, and in effect frozen to a particular dimensional state. Later, as the center portion of the glass cools, it wants to contract; however it is restrained to the dimensional state established by the outer layers of the glass body. This tendency to shrink by the inner portion of the glass places the outer zones of the glass in compression and the inner zone in tension. The tensile strength of the glass is generally improved by such techniques.
Similar techniques have been used in glass through the technique of ion exchange strengthening whereby large ions, such as potassium or sodium, are substituted in the surface of the glass in place of smaller ions, such as sodium or lithium, respectively. The overall effect is again to place the surface layers of the glass in compression which is balanced by an inner zone in tension so that the overall stress exhibited by the body is balanced. Techniques somewhat similar to this have been utilized in glass ceramics.
Classical ceramic bodies such as those which are oxides, carbides or nitrides of metals such as aluminum, berillium, silicon, titanium, zirconium and the like are formed into rigid bodies in a manner much different than that of glass. Ceramic bodies are generally crystalline while a "glass" body is typically amorphous, that is, noncrystalline, and is frequently considered to be a super-cooled liquid. Typically, in the formation of glasses and even of glass ceramics, at some stage in the processing a temperature is reached wherein the material is liquid or molten.
In the typical processing of ceramic bodies, no liquid condition is reached for all the components. Typically, a ceramic is formed by preparing a body of powdered material, such as aluminum oxide, and then forming it into some shape such as a tube, plate, bar, or other shape by slip-casting, powder-pressing, electrophoretic deposition, or other green-forming techniques. At one stage in such processing, the ceramic body is characterized as being in a green state, that is, the ceramic powders exist as a body in some physical shape and have some small strength provided by a binder or other means within the body. At this stage, the body is in form for firing or sintering whereby the body is fired to a temperature wherein glass diffusion occurs and the body becomes rigid and strong upon cooling. Such ceramic bodies, while having many advantages in terms of their refractoriness, are generally considered brittle, and generally fail in tension, even when subjected to compressive forces. The failure is generally due to tensile contact stresses.
Such ceramics generally have surface flaws which are often the sources of stress concentration and become sites of initiation of crack propogation which brings about ultimate failure of the ceramic body. One technique which has been utilized to strengthen such bodies is that of transformation toughening wherein certain crystalline materials are present metastably in the ceramic body in one crystal phase, for example, zirconia in the tetragonal crystal phase in the presence of a high stress field martensitically transformes into a monoclinic crystal which has a larger volume. This has been demonstrated in the art to terminate or retard the propogation of an existing crack, making the ceramic body tougher.
In recent work by Green, a sintered ceramic body of zirconia was heat treated to cause thermal diffusion of the stabilizing agent, yttria, to migrate to the surface of the body to depart the body by diffusion into another medium. The surface regions of the body then being depleted in yttria experience a phase transformation of zirconia particles from the tetragonal crystal form to the monoclinic crystal form thereby causing a compressive stress to form in the surface areas of the body because of the increased volume of the monoclinic crystals. Since the overall stress pattern of the body must be neutral, i.e. the stresses must be balanced, a corresponding tensile stress forms in the internal regions of the body.
The technique of Green is effective for forming compressive stresses in surface regions of sintered ceramic bodies. It is, however, somewhat limited as to the depth of compressive stress which can be formed since the operation is dependent upon thermally induced mass diffusion, which is quite slow. Also, because thermal diffusion occurs only at elevated temperatures, other competing phenomena proceed, such as grain growth of various crystals of the ceramic, which may be detrimental to overall strength. Also, there is no ready technique in the procedure of Green for selectively determining those regions in which a compressive stress will be induced.